Which level of protein structure describes the arrangement of multiple polypeptide chains forming a functional protein complex?

• A. Primary
• B. Secondary
• C. Quaternary
• D. Tertiary

Answer: (C)

Quaternary structure describes how multiple polypeptide chains come together to form a functional protein complex. Each chain is a subunit, and the way these subunits assemble and interact—through non-covalent forces or sometimes covalent bonds—defines the overall functional unit. A classic example is hemoglobin, which comprises four subunits that work together to transport oxygen.

In contrast, primary structure is simply the linear sequence of amino acids; secondary structure refers to local patterns like alpha helices and beta sheets stabilized by backbone hydrogen bonds; and tertiary structure is the three-dimensional shape of a single polypeptide chain, including how its side chains interact to create the overall fold. Since the focus here is on how multiple chains come together into one functional complex, this is characterized by quaternary structure.